The Gregorian calendar is the calendar in contemporary use in most countries.

The modern calendar began its existence in the Christian era, in 526, started by Pope John I. The chronologist of the Pope, Dionysius Exiguus, worked further on the calendar, especially concerning Easter.

During the Middle Ages, some problems were discovered with the use of the Julian calendar: every century had three to four days too many. In the sixteenth century the mistake grew to 10 days. Therefore, in 1582, it was decided that the calendar needed reform. Pope Gregory XIII decreed that October 4 should be followed by October 15 at once. Also, he decided that all of the leap days of the full century years which were not dividable by 400 would be omitted. In this manner, 1900 was not a leap year, 2000 was a leap year, and 2100 will not be.

The average duration of the Gregorian year is 365.2425 days. The diffirence with the real tropical year (365.2422) is so small that a new reformation will be needed in very, very distant future.

The Gregorian calendar was worked on by the Calabrian medicician Aloysius Lilius, as well as by the papal commissioner C. Clavius, before it reached the modern usage.

The new calendar came into use very slowly:

England and colonies (Northern America) in 1752;

Germany (as a whole) in 1776;

Sweden in 1823; and

Russia in 1918.

In The Netherlands the calendar took its start at different stages, in different provinces.

April Fool's Day

The historical record is not entirely clear, but it seems that April Fool's day was a direct result of the calendar change.
It began in 1582 in France. Before that year, the new year was celebrated for eight days, beginning on March 25. The celebration culminated on April 1. With the reform of the calendar under Charles IX, the Gregorian Calendar was introduced, and New Year's Day was moved to January 1.

However, communications being what they were in the days when news traveled by foot, many people did not receive the news for several years. Others, the more obstinate crowd, refused to accept the new calendar and continued to celebrate the new year on April 1. These backward folk were labeled as "fools" by the general populace. They were subject to some ridicule, and were often sent on "fools errands" or were made the butt of other practical jokes.

This harassment evolved, over time, into a tradition of prank-playing on the first day of April. The tradition eventually spread to England and Scotland in the eighteenth century. It was later introduced to the American colonies of both the English and French.